With an increasing prevalence of obesity in the U.S. comes increasing concern over the correlation of obesity and other health issues, such as hypertension, type 2 diabetes and heart disease. By tracking your weight status, you can take firmer control of your health and lower your risk. Two numbers that can help you assess your weight status are BMI and the body fat index.
BMI Definition
BMI stands for body mass index. It's a number describing a relationship between your height and your weight. To calculate your BMI, divide your weight in pounds by the square of your height in inches, then multiply the result by 703.
Interpreting BMI
The weight status categories of BMI are the same for all adults over 20 regardless of sex. If your BMI is between 18.5 and 24.9, you're in the normal category. Below that, you'd be considered underweight. A BMI of 25 to 29.9 belongs to the overweight category, and a BMI of 30 and above indicates obesity.
If you're younger than 20, your doctor plots your BMI onto a chart that takes into account for how body fat amount changes with age and varies between boys and girls. The healthy weight category spans from the 5th percentile to less than the to 85th percentile. Less than the 5th percentile is considered underweight. From the 85th to less than the 95th percentile is overweight while the 95th percentile and higher indicates obesity.
BMI Limitations
BMI does not directly measure body fat, nor does it take into account differences between adults' sexes or age ranges. It can't differentiate between bodies built to different proportions, ranges of skeletal density or amount of musculature. Thus your BMI is not a diagnosis but rather one indicator among many for assessing health risk.
Body Fat Index Definition
Your body fat index, or body fat percentage, is a more accurate indicator of your weight status category than BMI because it tracks what percentage of your total is actually fat. The rest of your body is lean body mass: bone, muscle, organ tissue, blood and other substances that aren't fat.
Interpreting Body Fat Index
Your body fat index maps to a series of categories indicating relative heath. According to the American Council on Exercise, a body fat index between 21 and 31 percent is acceptable for women, while men's body fat should fall between 14 and 25 percent.
Determining Your Body Fat Index
The most accurate method for measuring your body fat index, according to the Georgia State University Department of Kinesiology and Health, is underwater weighing. This method, also known as hydrostatic weighing, relies on knowing that fat floats while bone and muscle sink. This method is accurate to within 1.5 percent if performed correctly, but requires a lot of time, equipment and space to perform, in addition to an appointment with a specialist.
A more inexpensive method is to measure subcutaneous tissue with a skinfold caliper at either seven or three sites of the body. Another technique requires a scale, a tape measure and the mathematical formula that bodybuilding author Dave Draper provides on his website. Weigh yourself, measure the circumference of your waist. If you're a woman, also measure the circumference of your wrist, forearm and hip at the widest points. Plug these measurements into the men's or women's formula as appropriate to get your body fat index. Though neither of these methods is perfectly accurate, they are adequate to help you track your progress.
References
- Centers for Disease Control and Prevention: About Adult BMI
- Centers for Disease Control and Prevention: About BMI for Children and Teens
- HealthCheck Systems: Understanding Your Body Fat Percentage
- DaveDraper.com: Body Fat Calculator
- Georgia State University Department of Kinesiology and Health: Body Composition
- American Medical Association: Excess Deaths Associated with Underweight, Overweight, and Obesity
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